US5528051A - Quantum well infrared detector - Google Patents

Quantum well infrared detector Download PDF

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US5528051A
US5528051A US08/256,760 US25676094A US5528051A US 5528051 A US5528051 A US 5528051A US 25676094 A US25676094 A US 25676094A US 5528051 A US5528051 A US 5528051A
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Linh T. Nuyen
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035236—Superlattices; Multiple quantum well structures
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals

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  • This invention concerns semiconductor infrared detectors.
  • detectors have particularly advantageous applications in the range from 8 ⁇ m to 12 ⁇ m because these wavelengths correspond to a transparent window in the atmosphere.
  • this is a preferred sensitivity range the invention is not limited to this particular range of values.
  • the material used most often for these semiconductor detectors is the alloy HgCdTe but this material is metallurgically extremely complex, which makes its industrial production difficult.
  • One of the objects of the invention is to overcome this limitation, by providing a quantum well detector with III-V semiconductors which has a high detection sensitivity.
  • the detector of the invention which is of the type specified above, i.e. comprising a succession of alternating stacked layers of a III-V semiconductor material with a large forbidden band and a III-V semiconductor material with a small forbidden band with p-doping, defining a quantum well with sub-bands of HH and LH type in the region of the layer comprising the material with a small forbidden band in the valence band diagram of each corresponding heterostructure, is characterized in that the thickness of the material with a small forbidden band is essentially selected in such a manner that only two quantum sub-levels LH 1 and HH 1 appear in the well, and in that the energy difference between these two sub-levels corresponds to the energy of the photons to be detected, and the composition of the material with the large forbidden band is essentially selected in such a manner that the height of the barrier adjacent the quantum well is equal to or greater than the energy of the LH 1 sub-band.
  • the material with a large forbidden band its thickness is very advantageously so selected that the potential barriers defined by the layers of this material are sufficiently low for the resonant tunnel effect occurring through these barriers of light holes populating the sub-level LH 1 to create for these light holes a state in which the wave function thereof is spread in the assembly of the quantum wells and potential barriers, while that of the heavy holes populating the HH 1 sub-level is localized.
  • the material of this structure with a large forbidden band is preferably Al x Ga 1-x As and the material with a small forbidden band GaAs.
  • the thickness of the material with a small forbidden band lies in the range 1.5 nm to 2.5 nm approx., and the thickness of the material with a large forbidden band is about 8 nm approx.
  • the material with the small forbidden band can be In y Ga 1-y As, with an indium content Y In ⁇ 0.05 approx.
  • FIG. 1 is a schematic representation of the conduction band of a stack of AlGaAs/GaAs layers.
  • FIGS. 2a and 2b show the behavior of the conduction band of a prior art structure, respectively when quiescent and when biased, but with the thickness of the layers of GaAs reduced in such a manner that the energy difference of the quantum sub-bands corresponds to the wavelength of the light to be detected.
  • FIGS. 3a and 3b correspond to FIGS. 2a and 2b for the case in which the composition of the AlGaAs layer has been so selected that the higher quantum sub-level comes level with the edge of the quantum well.
  • FIG. 4 is a schematic representation of the valence band of a stack of AlGaAs/GaAs layers.
  • FIGS. 5a and 5b illustrate the behavior of the valence band of a prior art structure such as that of FIG. 4, respectively when quiescent and biased.
  • FIGS. 6a and 6b correspond to FIGS. 5a and 5b but for a structure in which the doping and dimensions of the layers have been determined in accordance with the teaching of the invention.
  • FIGS. 7a and 7b correspond to FIGS. 6a and 6b, for a modified embodiment of the invention.
  • These detectors are essentially formed by a stack of heterostructures 1, as illustrated in FIG. 1, each formed by a layer 2 of GaAs and a layer 3 of AlGaAs. These different layers are deposited epitaxially one on the other and the complete stack can comprise up to about fifty heterostructures 1. This arrangement of layers creates a corresponding succession of discontinuities, alternately quantum wells 4 and potential barriers 5, in the conduction band E c , shown schematically in FIG. 1.
  • quantum sub-bands quantum levels
  • E 1 , E 2 , E 3 , etc. appear from the quantum effect.
  • the thickness of the GaAs layers i.e. the width of the quantum wells 4
  • m* is the mass of an electron
  • ⁇ E is the height of the barrier for the electron in question ( ⁇ E 1 for the sub-band E 1 and ⁇ E 2 for the sub-band E 2 ),
  • d is the thickness of the potential barrier of AlGaAs
  • V is the applied voltage.
  • a first limitation in the performance of this structure is the low lifetime of the electrons in the structure, which is related to the lifetime of electrons in the material (AlGaAs) forming the potential barrier in which the hot electrons move.
  • a second limitation is similar to the phenomenon of inter-sub-band optical absorption which should obey certain rules of quantum mechanical selection prohibiting light incident perpendicular to the plane of the layers.
  • the optical inter-sub-band absorption can be obtained with light incident perpendicular to the plane of the layers.
  • the cited article thus refers to a detection sensitivity of 3.1 ⁇ 10 10 cm. Hz -1/2 /W at 77 K. for the wavelength 7.9 ⁇ m.
  • FIGS. 5a and 5b represent schematically the case described by Levine in the article last cited; it relates to quantum wells 9 which are 3 nm to 4 nm wide with a barrier height ⁇ E v in the order of 160 meV, for an aluminum content of 0.30 of the material AlGaAs. (Here and in the following the "content” is understood to be the mole fraction x Al of Al x Ga 1-x As).
  • the sub-band HH 2 is nearly at the edge of the well and the difference in energy between the edge of the well and the sub-band HH 1 is 144 meV for a well of 3 nm or 157 meV for a well of 4 nm (FIG. 5a).
  • This prior art structure has another problem in the presence of the sub-band LH 1 in the quantum wells 9.
  • this sub-band is populated with holes of low effective mass which are about 50 meV from the edge of the quantum well.
  • the tunneling transparency of these light holes is thus high, the more so because the Fermi level approaches the level LH 1 , i.e. the layer of GaAs is p-doped--which applies in this case with the doping lying between 10 17 cm -3 and 5 ⁇ 10 18 cm -3 approx.
  • the invention which will now be described, specifically seeks to overcome this problem. Its teaching lies essentially in an appropriate choice of the composition of the AlGaAs alloy and of the thicknesses of the layers of GaAs and AlGaAs.
  • the invention in particular used the phenomenon of inter-sub-band transition between HH 1 and LH 1 in the first place to eliminate the parasitic tunnel effect due to the light LH 1 holes.
  • the second place once this parasitic effect has been eliminated, it is possible to reduce the width of the AlGaAs barrier insofar as this is needed, because the holes populating the sub-level HH 1 are very heavy, exhibiting a low tunneling transparency and therefore only having a small effect on the performance of the detector.
  • this reduction of thickness can be by a factor at least equivalent to:
  • the detector For the detector to operate at the selected wavelength of about 10 ⁇ m it is necessary that the energy between the two sub-bands LH 1 and HH 1 shall be in the order of 124 meV. It can be shown by quantum mechanics calculations that, in this case, the width of the hole should be in the order of 1.5 nm to 2.5 nm, this thickness depending on the height of the potential barrier, i.e. on the aluminum content in the AlGaAs. The calculations show that the height of the barrier should exceed about 230 meV, which requires an aluminum content greater than about 0.42.
  • FIG. 6a at equilibrium
  • FIG. 6b under illumination and with an applied electric field
  • the quantum well detector of the invention is thus formed from a succession of quantum wells 9 of very small width (around 1.5 nm to 2.5 nm), separated by potential barriers 10 which are themselves relatively thin.
  • the LH 1 holes which have a low effective mass, can pass through thin barriers by the tunnel effect, even if the height of the barrier ⁇ E v is above their energy level, and this the more easily when these levels present the same energy, thus creating the well-known "resonant tunnel effect".
  • the holes populating the sub-band LH 1 thus come to move freely, perpendicular to the plane of the layers, in a periodic potential, sometimes also known as a "superlattice".
  • the sub-band LH 1 is quantized, it has its wave function completely spread over the whole structure, by the resonant tunnel effect, which leads to an increase ⁇ E LH1 in the energy of the sub-band, as can be seen at 13 in FIGS. 6a and 6b.
  • This increase ⁇ E LH1 depends on the width of the well. For wells 2 nm wide, the increase is in the order of 50 meV, 15 meV and 5 meV for barriers 5 nm, 7.5 nm and 10 nm wide respectively (the height of the barrier also having an effect on these values).
  • the light holes LH 1 moving in the periodic potential have a lifetime longer than that of the heavy holes moving in the continuum of the valence band, as explained with reference to FIG. 5.
  • a variation of an atomic monolayer i.e. in the order of 0.2 nm to 0.3 nm involves an relative variation in thickness of 10%. It is thus necessary to use techniques such as molecular beam expitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) which are compatible with such precision and ensure good uniformity of the epitaxy over the whole surface of the semiconductor chip.
  • MBE molecular beam expitaxy
  • MOCVD metal organic chemical vapor deposition
  • the invention is not limited to a GaAs/A1GaAs heterojunction and its teaching can apply equally to other heterostructures formed on the basis of III-V alloys.
  • the alloy InGaAs can be used in place of GaAs to realize the quantum wells.
  • this alloy InGaAs having a lattice constant greater than that of GaAs it will be subjected to a uniaxial strain which will have the effect of pushing the sub-band LH 1 further away in energy, in other words of increasing the energy difference between the sub-bands HH 1 and LH 1 . It is possible, for the same performance, to enlarge the quantum well by this phenomenon, relative to a similar structure using GaAs, and thus to facilitate practical implementation by virtue of a greater thickness of the layer to be deposited.

Abstract

The semiconductor component, comprises a succession of alternating stacked layers of a III-V semiconductor material with a large forbidden band such as Alx Ga1-x As and a III-V semiconductor material with a small forbidden band such as GaAs with p-doping, defining a quantum (9) with sub-bands of HH and LH type in the region of the layer comprising the material with a small forbidden band in the valence band diagram (Ev) of each corresponding heterostructure. According to the invention, the thickness of the material with a small forbidden band is essentially selected in such a manner that only two quantum sub-levels LH1 and HH1 appear in the well, and the energy difference between these two sub-levels corresponds to the energy of the photons (6) to be detected, and the composition of the material with the large forbidden band is essentially selected in such a manner that the height adjacent the barrier (ΔEv) of the quantum well is equal to or greater than the energy of the LH1 sub-band.

Description

This invention concerns semiconductor infrared detectors.
These detectors have particularly advantageous applications in the range from 8 μm to 12 μm because these wavelengths correspond to a transparent window in the atmosphere. However, although this is a preferred sensitivity range, the invention is not limited to this particular range of values.
The material used most often for these semiconductor detectors is the alloy HgCdTe but this material is metallurgically extremely complex, which makes its industrial production difficult.
Several laboratories have recently proposed using a new type of infrared detector--of the generic type to which the detector of the invention pertains--based on the principle of absorption of photons associated with transitions between two quantum sub-bands appearing in the quantum wells created by stacking a very large number of alternating epitaxial layers of III-V semiconductor material.
The advantage of these detectors comes mainly from the fact that the use of III-V compounds in place of II-VI compounds such as HgCdTe involves matellurgy that is much more manageable, making it easier to effect industrial production of these detectors.
However, for various reasons which are explained in detail below, the performance of these quantum well detectors with III-V semiconductors has so far been inferior to that of HgCdTe detectors.
One of the objects of the invention is to overcome this limitation, by providing a quantum well detector with III-V semiconductors which has a high detection sensitivity.
To this end, the detector of the invention, which is of the type specified above, i.e. comprising a succession of alternating stacked layers of a III-V semiconductor material with a large forbidden band and a III-V semiconductor material with a small forbidden band with p-doping, defining a quantum well with sub-bands of HH and LH type in the region of the layer comprising the material with a small forbidden band in the valence band diagram of each corresponding heterostructure, is characterized in that the thickness of the material with a small forbidden band is essentially selected in such a manner that only two quantum sub-levels LH1 and HH1 appear in the well, and in that the energy difference between these two sub-levels corresponds to the energy of the photons to be detected, and the composition of the material with the large forbidden band is essentially selected in such a manner that the height of the barrier adjacent the quantum well is equal to or greater than the energy of the LH1 sub-band.
As to the material with a large forbidden band, its thickness is very advantageously so selected that the potential barriers defined by the layers of this material are sufficiently low for the resonant tunnel effect occurring through these barriers of light holes populating the sub-level LH1 to create for these light holes a state in which the wave function thereof is spread in the assembly of the quantum wells and potential barriers, while that of the heavy holes populating the HH1 sub-level is localized.
The material of this structure with a large forbidden band is preferably Alx Ga1-x As and the material with a small forbidden band GaAs. In this case, the thickness of the material with a small forbidden band lies in the range 1.5 nm to 2.5 nm approx., and the thickness of the material with a large forbidden band is about 8 nm approx. In a variant, the material with the small forbidden band can be Iny Ga1-y As, with an indium content YIn≦ 0.05 approx.
The invention is now explained in more detail, with reference to the accompanying drawings. In all of the figures the same reference numerals designate like parts.
FIG. 1 is a schematic representation of the conduction band of a stack of AlGaAs/GaAs layers.
FIGS. 2a and 2b show the behavior of the conduction band of a prior art structure, respectively when quiescent and when biased, but with the thickness of the layers of GaAs reduced in such a manner that the energy difference of the quantum sub-bands corresponds to the wavelength of the light to be detected.
FIGS. 3a and 3b correspond to FIGS. 2a and 2b for the case in which the composition of the AlGaAs layer has been so selected that the higher quantum sub-level comes level with the edge of the quantum well.
FIG. 4 is a schematic representation of the valence band of a stack of AlGaAs/GaAs layers.
FIGS. 5a and 5b illustrate the behavior of the valence band of a prior art structure such as that of FIG. 4, respectively when quiescent and biased.
FIGS. 6a and 6b correspond to FIGS. 5a and 5b but for a structure in which the doping and dimensions of the layers have been determined in accordance with the teaching of the invention.
FIGS. 7a and 7b correspond to FIGS. 6a and 6b, for a modified embodiment of the invention.
The current state of quantum well detectors is firstly recalled, especially the mechanism whereby these detectors operate.
These detectors are essentially formed by a stack of heterostructures 1, as illustrated in FIG. 1, each formed by a layer 2 of GaAs and a layer 3 of AlGaAs. These different layers are deposited epitaxially one on the other and the complete stack can comprise up to about fifty heterostructures 1. This arrangement of layers creates a corresponding succession of discontinuities, alternately quantum wells 4 and potential barriers 5, in the conduction band Ec, shown schematically in FIG. 1.
If the GaAs layers are thin enough, in the order of a few nanometers, quantum sub-bands (quantum levels) E1, E2, E3, etc. appear from the quantum effect. By appropriate selection of the thickness of the GaAs layers, i.e. the width of the quantum wells 4, it is possible to adjust the energy positions of the E1 and E2 sub-bands in such a manner that the difference ΔE1 -ΔE2 between the energies of the quantum levels E2 and E1 will be in the order of 124 meV approx., as illustrated in FIG. 2a. If such a structure is illuminated by light with a wavelength of 10 μm, i.e. by photons with energy h=124 meV, these photons will induce a resonant electronic transition from the sub-band E1 to the sub-band E2, symbolized by the arrow 7. If an electric field is applied to such a structure (FIG. 2b), that is to say if the component is biased, the electrons in the sub-band E2 can pass by the tunnel effect through the potential barriers 5 corresponding to the AlGaAs layers, as symbolized by the arrow 8, thus generating a measurable photo-current.
The first detectors implemented on this principle have been described by B. F. Levine et al., New 10 μm Infrared Detector Using Intersubband Absorption in Resonant Tunneling GaA1As Superlattices, Appl. Phys. Lett., Vol. 50, No. 16, p. 1092 (1987). Account is taken there of the incidence of a parasitic tunnel current due to electrons in level E1 whose tunneling transparency, although less than that of the electrons in level E2 cannot be neglected. This tunnel current thus creates a high dark current which is detrimental to the performance of the detector.
In this respect, it is known that the tunneling transparency is expressed by an equation of the type:
T=A exp[-(m*.sup.1/2 ΔE.sup.3/2 d)/V],               (1)
where:
m* is the mass of an electron,
ΔE is the height of the barrier for the electron in question (ΔE1 for the sub-band E1 and ΔE2 for the sub-band E2),
d is the thickness of the potential barrier of AlGaAs, and
V is the applied voltage.
It has then been proposed to increase the thickness d of the barrier and/or to increase its height ΔE1. However, doing this also reduces the tunneling transparency of the electrons in the sub-band E2, i.e. those which generate the photo-current. A compromise appears to be difficult to achieve and the quantum well detectors with the best performance currently made are designed in such a manner that the E2 sub-band is located just at the edge of the well, as seen in FIGS. 3a and 3b,
It is thus possible to increase the thickness of the potential barrier 5 or in other words reduce the dark current without however affecting the photo-current. Reference may be made in this respect to the work of B. F. Levine et al., Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors, J. Appl. Phys., Vol. 64, No. 3, p. 1591 (1988), which refers to a "detectivity" D*=1010 cm. Hz-1/2 /W at 77 K., a result close to but still less than that obtained with a conventional HgCdTe detector, which yields 3×1010 cm. Hz-1/2 /W at the same temperature.
A first limitation in the performance of this structure is the low lifetime of the electrons in the structure, which is related to the lifetime of electrons in the material (AlGaAs) forming the potential barrier in which the hot electrons move.
A second limitation is similar to the phenomenon of inter-sub-band optical absorption which should obey certain rules of quantum mechanical selection prohibiting light incident perpendicular to the plane of the layers.
Very recently, this second difficulty has been tackled by B. F. Levine et al., Normal Incidence Hole Intersubband Absorption Long Wavelength GaAs/Alx Ga1-x As Quantum Well Infrared Photodetectors, Appl. Phys. Lett., Vol. 59, No. 15, p. 1864 (1991), which proposes to use quantum wells created by discontinuities in the valence band, not in the conduction band.
Thus, in the latter case, the optical inter-sub-band absorption can be obtained with light incident perpendicular to the plane of the layers. The cited article thus refers to a detection sensitivity of 3.1×1010 cm. Hz-1/2 /W at 77 K. for the wavelength 7.9 μm.
However, this proposal also suffers from limitations, due on this occasion to the low lifetime of the holes.
Thus, if we consider, as is illustrated in FIG. 4, the discontinuities created in the valence band of the structure, which alternates quantum wells 9 with potential barriers 10, it is apparent that in each quantum well 9 there are many sub-bands HH1, HH2, LH1, etc., (the designation HH referring to the heavy holes and LH to the light holes), Three of these sub-bands are illustrated in FIG. 4 but the situation can vary very considerably, depending on the width of the quantum wells 9 or the height ΔEv of the potential barrier 10.
FIGS. 5a and 5b represent schematically the case described by Levine in the article last cited; it relates to quantum wells 9 which are 3 nm to 4 nm wide with a barrier height ΔEv in the order of 160 meV, for an aluminum content of 0.30 of the material AlGaAs. (Here and in the following the "content" is understood to be the mole fraction xAl of Alx Ga1-x As). In this case, the sub-band HH2 is nearly at the edge of the well and the difference in energy between the edge of the well and the sub-band HH1 is 144 meV for a well of 3 nm or 157 meV for a well of 4 nm (FIG. 5a).
Under illumination (FIG. 5b), the photons with corresponding energy (144 meV and 157 meV respectively) are absorbed (arrow 11), thus causing movement of the holes in the continuum of the valence band (arrow 12) when an electric field is applied.
Since it is known that the effective mass of holes in the continuum of the valence band of AlGaAs (and GaAs) is very high, in the order of 0.4 mo (mo being the mass of the electron), it will be understood that their mobility will be reduced and that their mean free path will be short, in other words, their lifetime will be very short.
This prior art structure has another problem in the presence of the sub-band LH1 in the quantum wells 9. Thus this sub-band is populated with holes of low effective mass which are about 50 meV from the edge of the quantum well. Considering equation (1) above, the tunneling transparency of these light holes is thus high, the more so because the Fermi level approaches the level LH1, i.e. the layer of GaAs is p-doped--which applies in this case with the doping lying between 1017 cm-3 and 5×1018 cm-3 approx. Once again an irreconcilable compromise is encountered between a large barrier width, necessary to reduce the dark current which would otherwise be very high, and a concomitant reduction of the photo-current and thus of the sensitivity of the detector.
The invention, which will now be described, specifically seeks to overcome this problem. Its teaching lies essentially in an appropriate choice of the composition of the AlGaAs alloy and of the thicknesses of the layers of GaAs and AlGaAs.
The invention in particular used the phenomenon of inter-sub-band transition between HH1 and LH1 in the first place to eliminate the parasitic tunnel effect due to the light LH1 holes. In the second place, once this parasitic effect has been eliminated, it is possible to reduce the width of the AlGaAs barrier insofar as this is needed, because the holes populating the sub-level HH1 are very heavy, exhibiting a low tunneling transparency and therefore only having a small effect on the performance of the detector.
If equation (1) is applied, this reduction of thickness can be by a factor at least equivalent to:
(m*.sub.HH1 /m*.sub.LH1).sup.1/2 =2.4 approx.
and at most equivalent to
(m*.sub.HH1 /m*.sub.LH1).sup.1/2 (ΔE.sub.HH1 /ΔE.sub.LH1)3/2=12 approx.
where m*HH1 and m*LH1 are taken to be 0.4 mo and 0.07 mo respectively (mo being the mass of the electron), and ΔEHH1 and ΔELH1 are the heights of the respective barriers associated with the sub-bands HH1 and LH1, with ΔEHH1 =190 meV approx. and ΔELH1 =66 meV approx.
For the detector to operate at the selected wavelength of about 10 μm it is necessary that the energy between the two sub-bands LH1 and HH1 shall be in the order of 124 meV. It can be shown by quantum mechanics calculations that, in this case, the width of the hole should be in the order of 1.5 nm to 2.5 nm, this thickness depending on the height of the potential barrier, i.e. on the aluminum content in the AlGaAs. The calculations show that the height of the barrier should exceed about 230 meV, which requires an aluminum content greater than about 0.42.
A structure in accordance with the invention is illustrated schematically in FIG. 6a (at equilibrium) and FIG. 6b (under illumination and with an applied electric field).
The quantum well detector of the invention is thus formed from a succession of quantum wells 9 of very small width (around 1.5 nm to 2.5 nm), separated by potential barriers 10 which are themselves relatively thin.
It is known that the LH1 holes, which have a low effective mass, can pass through thin barriers by the tunnel effect, even if the height of the barrier ΔEv is above their energy level, and this the more easily when these levels present the same energy, thus creating the well-known "resonant tunnel effect". The holes populating the sub-band LH1 thus come to move freely, perpendicular to the plane of the layers, in a periodic potential, sometimes also known as a "superlattice".
It is thus possible to be released from too precise a definition of the thickness of the AlGaAs by using barrier heights such that the sub-band LH1 is located right at the edge of the quantum well, as shown in FIGS. 7a and 7b; in this case, the height ΔEv of the barrier adjacent the quantum well is no longer greater than but just equal to the energy of the sub-band LH1. This configuration is close to that shown in FIGS. 5a and 5b (corresponding to the proposal of Levine in the cited article) but with the essential difference of the presence, in the prior art situation, of the LH1 sub-band located in the quantum well below the energy of the potential barrier, contributing thus to an increase in the dark current.
It is also known that although the sub-band LH1 is quantized, it has its wave function completely spread over the whole structure, by the resonant tunnel effect, which leads to an increase δELH1 in the energy of the sub-band, as can be seen at 13 in FIGS. 6a and 6b. This increase δELH1 depends on the width of the well. For wells 2 nm wide, the increase is in the order of 50 meV, 15 meV and 5 meV for barriers 5 nm, 7.5 nm and 10 nm wide respectively (the height of the barrier also having an effect on these values).
As for the holes populating the sub-band HH1, which do not only have a large effective mass but which are also located deep in the hole, these do not experience the tunnel effect and their wave function also remains localized.
In summary, the light holes LH1 moving in the periodic potential have a lifetime longer than that of the heavy holes moving in the continuum of the valence band, as explained with reference to FIG. 5.
Moreover, although the movement of holes in the band LH1 will be favored by a small barrier width (a small thickness of the AlGaAs layer), it is desirable not to reduce this value, because too large an increase in ΔELH1 will result in a reduction in the energy difference between the sub-bands HH1 and LH1, so increasing the dark current.
Knowing however that infrared detectors operate at the temperature of liquid nitrogen, namely 77 K., and that the thermal energy is 6.6 meV at this temperature, it is only important that the increase ΔELH1 shall be less than 6.6 meV. It is thus possible to find an optimum barrier width, defined in such a manner that ΔELH1 =6.6 meV approx. For wells 2 nm wide, this optimum corresponds to a thickness of AlGaAs of about 8 nm (a value which can vary with the aluminum content of the AlGaAs).
Taking into account the very low values of the width of the quantum wells which have been indicated, a variation of an atomic monolayer, i.e. in the order of 0.2 nm to 0.3 nm involves an relative variation in thickness of 10%. It is thus necessary to use techniques such as molecular beam expitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) which are compatible with such precision and ensure good uniformity of the epitaxy over the whole surface of the semiconductor chip.
Moreover, the invention is not limited to a GaAs/A1GaAs heterojunction and its teaching can apply equally to other heterostructures formed on the basis of III-V alloys.
In particular the alloy InGaAs can be used in place of GaAs to realize the quantum wells. In particular, this alloy InGaAs having a lattice constant greater than that of GaAs, it will be subjected to a uniaxial strain which will have the effect of pushing the sub-band LH1 further away in energy, in other words of increasing the energy difference between the sub-bands HH1 and LH1. It is possible, for the same performance, to enlarge the quantum well by this phenomenon, relative to a similar structure using GaAs, and thus to facilitate practical implementation by virtue of a greater thickness of the layer to be deposited.
This effect is particularly marked: thus, for an indium content (i.e. a mole fraction YIn of Iny Ga1-y As) of only 0.03, thus for a composition very close to that of GaAs, the increase in the energy is in the order of 15 meV. It is however noted that alloys with an indium content greater than 0.05 cannot really be used, because the excessive strain then risks creation of interfacial dislocations, given the total thickness of the quantum well structure.

Claims (7)

I claim:
1. A semiconductor component of quantum well infrared detector type comprising a succession of alternating stacked layers of III-V semiconductor material with a large forbidden band and a III-V semiconductor material with a small forbidden band with p-doping, defining an assembly of potential barriers alternating in series with quantum wells, each well including sub-bands of HH and LH type in the region of the layer comprising the material with the small forbidden band in the valance diagram (Ev) of each corresponding heterostructure, characterized in that:
the thickness of the layers of the material with a small forbidden band is selected such that only two quantum sub-levels LH1 and HH1 appear in each well, and in that the energy difference between the two sub-levels corresponds to the energy of photons to be detected, and
the material with the large forbidden band comprises a composition that causes the height (ΔEv) of the potential barriers to be equal to or greater than the energy of the LH1 sub-band.
2. The component of claim 2 wherein, for resonant-tunnel-effect light holes populate the sub-level LH1 and resonant-tunnel-effect heavy holes populate the HH1 sub-level, the thickness of the layers of the material with a large forbidden band is so selected that the height of the potential barriers defined by the layers of said material creates for the light holes a state in which light-hole wave function is spread over the assembly, while heavy-hole wave function is localized.
3. The component of claim 1 wherein the material with a large forbidden band is Alx Ga1-x As.
4. The component of claim 3 wherein the material with a small forbidden band is GaAs.
5. The component of claim 4 wherein the thickness of the layer of the material with a small forbidden band lies approximately between 1.5 nm and 2.5 nm.
6. The component of claim 4 wherein the thickness of the layers of the material with a large forbidden band is approximately 8 nm.
7. The component of claim 3 wherein the material with a small forbidden band is Iny Ga1-y As with an indium content of approximately YIn ≦0.05.
US08/256,760 1992-01-22 1993-01-21 Quantum well infrared detector Expired - Fee Related US5528051A (en)

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FR9200667A FR2686456A1 (en) 1992-01-22 1992-01-22 INFRARED DETECTOR WITH QUANTUM WELLS.
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PCT/FR1993/000060 WO1993015525A1 (en) 1992-01-22 1993-01-21 Quantum well infrared detector

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US5751830A (en) * 1995-05-24 1998-05-12 Lockheed Martin Energy Systems, Inc. Method and apparatus for coherent imaging of infrared energy
US6292089B1 (en) * 1996-01-11 2001-09-18 Imc Industriellt Mikroelektronikcentrum Ab Structures for temperature sensors and infrared detectors
US5978399A (en) * 1996-12-10 1999-11-02 Doughty; Kathryn L. Electrically-tunable infrared detectors and method based on intraband transitions in quantum well structures
WO1999019912A1 (en) * 1997-10-16 1999-04-22 California Institute Of Technology Dual-band quantum-well infrared sensing array
US6184538B1 (en) 1997-10-16 2001-02-06 California Institute Of Technology Dual-band quantum-well infrared sensing array having commonly biased contact layers
US6198101B1 (en) 1998-09-08 2001-03-06 Lockheed Martin Corporation Integral charge well for a QWIP FPA
US6271537B1 (en) 1998-11-20 2001-08-07 California Institute Of Technology Slotted quantum well sensor
US7211820B2 (en) 1998-11-20 2007-05-01 California Institute Of Technology Quantum well sensor having spatially separated sensing columns
WO2000031802A1 (en) * 1998-11-20 2000-06-02 California Institute Of Technology Slotted quantum well sensor
US6423980B1 (en) 1999-06-25 2002-07-23 California Institute Of Technology Multi-directional radiation coupling in quantum-well infrared photodetectors
US7253432B2 (en) * 2000-10-16 2007-08-07 Georgia State University Research Foundation, Inc. Heterojunction far infrared photodetector
US20050258415A1 (en) * 2000-10-16 2005-11-24 Perera A G U Heterojunction far infrared photodetector
US20040179198A1 (en) * 2003-03-14 2004-09-16 Serna Mario A. Polarimeter using quantum well stacks separated by gratings
US6906800B2 (en) 2003-03-14 2005-06-14 The United States Of America As Represented By The Secretary Of The Air Force Polarimeter using quantum well stacks separated by gratings
US20060091284A1 (en) * 2004-10-20 2006-05-04 Viens Jean F Multi-spectral pixel and focal plane array
US7566942B2 (en) 2004-10-20 2009-07-28 Massachusetts Institute Of Technology Multi-spectral pixel and focal plane array
US20070158638A1 (en) * 2005-10-21 2007-07-12 Perera A G U Dual band photodetector
US7838869B2 (en) 2005-10-21 2010-11-23 Georgia State University Research Foundation, Inc. Dual band photodetector
US20110049566A1 (en) * 2005-10-21 2011-03-03 Georgia State University Research Foundation, Inc. Dual Band Photodetector
US8093582B2 (en) 2005-10-21 2012-01-10 Georgia State University Research Foundation, Inc. Dual band photodetector
US20090020700A1 (en) * 2007-07-17 2009-01-22 Locheed Martin Corporation Method and device for generating an electrical signal in response to light

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WO1993015525A1 (en) 1993-08-05

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